Rarely mentioned, but Russia also tested the security of their (mobile signal networks) SS7 and Diameter systems Dec 16-17.
It never fails to amaze me how little attention is paid to these systems that can track, intercept and hijack the comms of mobile phone users of any country, internationally.
You could MITM Signal if you control the cell towers on either end. It would be visible, because the security number would change. But in my experience helping people set it up, very few users understand what those numbers are for, and what it means when they change. Most just mark them as verified immediately.
It never fails to amaze me how little attention is paid to these systems that can track, intercept and hijack the comms of mobile phone users of any country, internationally.